we use (must) / mustn't and have to + verb to talk about rules and obligations
you must/have to be quiet in the library. we mustn't use our mobile phones at school
we use don't have to+verb when an action is not necessary
we don't have to go to school tomorrow
we usually use have to not must in questions
Do we have to get up early tomorrow
the past of must and have to is had to
we had to help Dad in the garden yesterday